1. Field of Invention
This invention relates generally to baking utensils, and more particularly to a utensil adapted to operate in a microwave oven to convert microwave energy to which the utensil is exposed to thermal energy for cooking the food contained therein, and more particularly to a safety container for protectively housing a utensil of this type so that when cooking is completed it cannot be removed from the container until the utensil has cooled to a temperature level at which it is safe to handle.
2. Status of Prior Art
Baking is the process of cooking food with dry heat. Heat is transferred to the food by convection, conduction or infrared radiation, depending on the nature of the oven. But in all cases, thermal energy is applied to the outer surface of the food and by reason of heat transfer from this surface to the inner body of the food, cooking takes place from the outside in. As a consequence, as cooking continues, the surface of the food may become scorched or browned. Because this action usually renders the food more palatable, such surface scorching or browning is normally regarded as desirable.
The use of a microwave oven to heat or cook food is commonplace, and microwave ovens are now installed in many households. In the typical microwave oven, a magnetron functions to generate microwave energy at a frequency of about 1000 mHz. This energy is conveyed by a wave guide to the interior of the oven to irradiate the food placed therein. Because food more or less absorbs microwave energy, this gives rise to internal molecular friction which heats the food at a rate that depends on its "lossy" characteristics. Some food products are heated more rapidly than others in a microwave oven; but in general the cooking of food by microwave energy is much faster than by conventional heating techniques, including infrared radiation.
In heating or cooking food in a microwave oven, the food is placed in a receptacle of synthetic plastic, glass or other material which is non-reactive to microwave energy; hence, it is only the food that is heated. U.S. Pat. Nos. 4,703,149 and 4,416,906 disclose microwave food heating utensils. In some cases, as pointed out in U.S. Pat. No. 4,416,906, microwave cooking of food is uneven because of dry spots in some areas of the food and moist spots in other areas.
Cooking takes place in a microwave oven throughout the body of the food; hence browning or scorching of the outer surface or crust does not occur. Thus if one sought to bake a loaf of bread in a microwave oven it would have no crust. This is a recognized disadvantage of microwave cooking.
U.S. Pat. No. 3,941,967 discloses a microwave cooking apparatus capable of scorching the surface of the food being cooked without excessively heating the interior of the food. This apparatus, which is designed to be put into a microwave oven, takes the form of a casing within which is disposed a plate on which the food to be cooked is placed. Below the plate is a thermal heating element which generates heat by absorption of microwave radiation, use for this purpose being made of a ferrite heating element. Thus, the interior of the food is heated by the microwave energy absorbed thereby, while at the same time the exterior of the food is thermally heated and scorched by the plate heated by the ferrite heating element.
A similar arrangement is shown in U.S. Pat. No. 4,496,815, in which a microwave browning utensil includes a metal platter on which the food to be heated in the microwave oven is placed. On the underside of the platter is a thermal heating element formed by powdered ferrites dispersed in a matrix. In this way, the interior of the food on the platter which absorbs microwave energy is heated and cooked thereby, while its exterior is thermally heated and browned. Thermal heating takes place mainly by conduction; hence, the exterior of a body being heated is first subjected to the heat before it penetrates the interior of the food body, whereas with microwave heating, the radiation penetrates the interior of the body.
U.S. Pat. No. 4,266,108 discloses the use of ferrite material adjacent a microwave reflecting member in which the ferrite material acts as a heating element that will rise in temperature to a predetermined level which depends on the Curie point of thee ferrite. U.S. Pat. No. 4,664,506 discloses an aluminum microwave cooking vessel having discrete layers of ferrite particles bonded thereto to convert microwave energy to thermal energy. When in its ferromagnetic state, the ferrite then absorbs microwave energy and is heated thereby. This action ceases when the ferrite enters its paramagnetic state. Hence, when a ferrite heating element is placed in a microwave oven and is subjected to microwave energy, the heating element will become increasingly hot until an elevated temperature is reached that depends on the Curie point of the ferrite, after which no more heat is generated even though the microwave oven is still operating. Thus, the ferrite heating element will effectively be turned "off," even though the microwave oven is still "on."
Our above-identified application, Ser. No. 08/173,784 discloses a baking utensil adapted to operate in a microwave oven to convert microwave energy to which the utensil is exposed into thermal energy for cooking food contained therein. The utensil is composed of complementary, thermally-conductive upper and lower sections, each having bonded to its outer surface an epoxy matrix layer. Dispersed throughout the matrix layer are ferrite particles which absorb microwave energy to produce thermal energy that is conducted by the sections of the utensil to the food enveloped thereby. The Curie point of the ferrite particles is such as to arrest their absorption of microwave energy when the utensil temperature approaches a level which is excessive for the food product being cooked or baked.
One who puts this utensil in a microwave oven does not exactly know how long it take for baking or cooking of food to be completed and therefore sets the microwave oven to turn off after a time period that is beyond the estimated cooking time. However, no danger exists that the food will be overcooked, for the Curie point of the ferrite particles is such as to cut off thermal heating when the utensil temperature exceeds a predetermined level. However, when cooking is completed and the utensil is to be removed from the microwave oven, the person who then removes the utensil from the oven runs the risk of being burned, for the utensil is still very hot.
In our above-identified copending application Ser. No. 08/173,784 there is disclosed a safety container for a mold assembly for use by children to produce in a microwave oven shaped plastic play pieces, the assembly including a ferrite heating element responsive to microwave energy which acts to thermally heat the plastic material held in the mold assembly. This safety container which houses the mold assembly includes a removable cover which is latched when the heating element raises the temperature in the oven to a high level, and is automatically unlatched when the temperature is relatively cool so that the child can then safely remove the mold assembly from the container.